Current transformer



May 23, 1944. K. wE'rHMULLER CURRENT TRANSFORMER Filed Aug. 1, 1941Patented May 23, 1944 CURRENT TRANSFORMER Karl Wethmiiller,Berlin-Baumschulenweg, Germany, assigner to General Electric Company, acorporation of New York Application August 1, 1941, Serial No. 405,091In Germany March 29, 1940 5 Claims. (Cl. 175-358) The present inventionrelates to transformers and more particularly to current transformersfor high voltage application.

In a well known and frequently employed design for high tension currenttransformers, the high tension insulation of the primary winding isformed of a U-shaped insulating body, usually of porcelain, whichconsists principally of `a tube of round or rectangular cross sectionopen at both ends of the two legs, and in the passage of which tube theprimary winding is arranged. While this type of insulation is desirablefrom an electrical standpoint, its mechanlcal strength is low so thatspecial precautions must be taken in order that the insulating body willbe able to withstand short circuit stresses and strains imposed thereonthrough the support or the primary terminal lugs. To provide forsufficient mechanical strength, special metal bracing means and baseshave usually been employed but such arrangements inherently result in a'high cost unit.

It is an object of the present invention to provide a new and improvedtransformer cf the type referred to which is mechanically strong and lowin manufacturing cost.

It is a further object of the present invention to provide a currenttransformer suitable for use in high voltage circuits having the primarywinding arranged within a tubular body of :1

molded insulating material which is so constructed and arranged that itwill be able to withstand stresses resulting from short circuitconditions of primary current.

It is a still further object of the invention to :t

provide a new and improved current transformer of the type referred tohaving a magnetic core structure so arranged aro und the insulating bodyas to reinforce the same against stresses arising from the enclosedwinding.

Another object of the present invention is to provide a new and improvedcurrent transformer having an insulating body of the type referred towhich may be readily secured to a suitable support without imposingsevere strains upon the insulating body. A

A further object of the invention is to provide a current transformer ofthe type referred to having a new and improved primary terminalstructure.

In accordance with one form of the invention, the U-shaped tubularinsulating body, within which the primary winding is arranged, has anintegral web connecting together the upper ends of the U legs forstrengthening these portions against the outwardly directed forcesarising from short circuit primary current conditions. The primarywinding terminals have tubular portions extending into the upper openends of the insulating body so as to distribute the stresses which maybe transmitted through the terminals. The magnetic core structure issnugly clamped or cemented against the lower portion of the insulatingbody in a manner as to reinforce these portions against the likelihoodof breakage under short circuit stresses. The lower portion of theinsulating body is furthermore provided with an integral base structurewhich is so constructed and arranged that the transformer may be readilymounted upon a suitable support in a manner so that no strains likely tocause breakage of the insulating body may be transmitted through thesupport to the transformer proper.

Further objects and advantages oi' the invention will appear from thefollowing description and the claims appended thereto taken inconnection with the accompanying drawing.

In the drawing Fig. 1 is a view, halfin longitudinal section and half inelevation, of a transformer constructed in accordance with one form ofthe invention; Fig. 2 is a View of the transformer showing `half of itin cross section along the line A-B of Fig. 1 and half of it in a planview; and Fig. 3 is a fragmentary view illustrating a modification ofthe invention.

The transformer comprises a U-shaped tubular body of a suitable solidinsulating material, such as porcelain, having anintcgral base portion2. If the body is made of porcelain, it can be molded as a unit, or theU-shaped part and the base part may be molded separately and then bejoined together prior to the baking of the porcelain. The insulatingbody has between its two legs a strong connecting web 3 which braceseffectively the legs with respect to each other, and which constitutesan integral part of the insulating body, so that the parts l, 2 and 3form a single unit. Within the insulating body a passage, preferably ofrectangular cross section, is provided which follows the curve of the U,and through which extends the primary winding 4` which may be introducedinto the open ends of the U legs. In order to facilitate theintroduction of the winding, the cross section of the winding space inthe U curve may be increased as indicated at 5 in Fig, 1. The primarywinding is provided with terminal lugs 6 which, for the purpose ofeffectivey fastening to the insulator even Without cement-ing, areprovided with tubular extensions 0' which plane of the laminations.

are inserted into the ends of the legs. 'I'he upper ends of theA U legsare suitably enlarged for cooperatively receiving these tubularextensions. It is advantageous to place between the inserted tubularparts of the terminals and the inner walls of the insulator, yielding orelastic intermediate layers l, such as pressboard, which permit/atlghtiltting of the terminals and will prevent metal from restingagainst the` solid insulating material of the U body. 'I'his arrangementwill distribute over a considerable area of the insulating body anystrainsv which may be transmittedthrough the leads and reinforce thetransformer against stresses caused by the winding in the case of shortcircuit. As illustrated by the modication of Fig. 3, both terminal lugs6 may be secured to a common support 8 of a suitable insulating materialand which support is provided with a pair of tubular extensions 3' forinsertion into the ends of the U legs. This sup porting member, whichmay be suitably stiifened, will increase the effect of the cross member3 that braces the two legs of the insulator and will increase themechanical strength of the transformer against short circuit and. otherstresses. Both the primary terminals and the winding are covered by alightweight cap 3 which is preferably made of molded insulating materialand which is horizontally divided to facilitate assembly thereof.

The laminated iron core I of the current transformer is arranged betweenthe base 2 and the beads II of the insulating bushings. It is,therefore, supported directly by the insulating body and compressedthereby at right angles to the In the physical embodiment, the corecomprises two equal stacks, each of which surrounds a leg of theinsulator and consists of U-shaped laminations I2 and straight yokestrips I2' which are alternately stacked. However, other methods can beapplied for making the core; for instance, the core may be built as ashell constituting a single part, which is made up of E-shapedlaminations and straight yoke pieces. The spool I3 for the secondarycoil with the secondary winding Il lies in the center between the two U`legs of the insulating body and surrounds the two central legs of thecore stacks. In the case of a shell-type transformer, it surrounds thecenter leg. The leads I5 of the secondary winding extend into thebasepart 2 of the insulator and are connected to the secondary terminals I6which are directly secured to the base, and which may be protected by asuitable cover. The core may be coveredby a metal band, for instance aniron band I'I, the ends of.which can be fastened together by means ofscrews I8. A terminal I9 may be secured .to the band L1 for groundingthe core. If the outer surface of the U part ofthe insulator is given aconductive coating, a well known procedure, for the purpose ofeliminating the dielectric stresses set up by the high-tension field inthe surrounding air, it is advisable that the slightly raised'surfaces20 be also provided with the same conducting coating and, in thismanner, the latter can be easily grounded without it being necessary toadopt special means. The surfaces of the beads I0 of the bushingscontacting the core may also be coated for the same purpose.

In the event of a short circuit, the primary winding exerts an outwardforce -mainly against the parts of the insulator wall which areindicated at 2 I. In order to give the insulator maximum mechanicalstrength against short circuits,

it is advisable to brace these parts of the wall particularly well. Inaccordance with the invention, cement 23 is injected, for instance fromthe bottom of the base through the openings 22 provided for thatpurpose, between the insulating body and the lamination stacks so thatthe core has a good, positive, mechanical contact with the parts of thewall that are to be braced.

Adjacent legs of the two lamination stacks 1o are secured together bymeans of clamping plates 24 and bolts 25 on opposite sides of theinsulator. With this construction any short circuit forces arising inthe primary winding, which react externally, are effectively absorbed bythe iron core,

and a considerable mechanical strength against short circuit stresses isattained. However, other arrangements may be employed to obtain a rigidconnection between the stack of laminations and' the walls of theinsulating body which are to be braced. For example, the laminationstacks.

an which are above the core and which are not supported by the latter,are protected against the forces set up in the primary winding byrecesses 20, so that at these points the winding is not in contact withthe insulator and the short circuit forces are transmitted to otherparts of the wall that are supported by the core.

'I'he base 2 of the insulating body may be utilized for fastening thetransformer to a suitable support. For this purpose, bolt holes 21 maybe provided in the base. The holes are preferably made elongate in crosssection in order to allow for manufacturing tolerances which may benecessitated due to the properties of the insulation material used. Inthe case of brittle insulating material such as porcelain it is notdesirable that the bolts and nuts engage directly with and tightlyagainst the insulator for the reason that localized strains might causea breakage of the insulator. In order to prevent this effectively, steeltubings or bushings 28 are provided in the holes. These steel tubingsare riveted at their protruding ends, with suitable elastic washers 29,such as pressboard, fiber, rubber, etc. and metal washers 30, whoseholes may be suitably beveled, inserted between the insulating body andthe steel tubing prior to the riveting of the latter. In this manner thesteel tubing is secured firmly in the bolt holes without exerting anyexcessive pressure on the insulating body. The elastic or yielding disks29 lie between the insulating body and the metal washers 30 so that nometal contacts the porcelain. As regards the bolts, these are passedthrough the bushings and, when they are tightened, their pressure istaken up by the bushings and not by the insulating body. As a result,the mechanical strength of the latter need not be considered, and thebolts may be tightenel considerably.

If in the conventional four-point support of transformers the supportingbase is not quite level, or if the supporting surfaces, e. g. washers30, of the transformer are not exactly in one plane, any tightening ofthe bolts would create mechanical stresses between the points ofsupport, which, in the case of a brittle material such as porcelain,will crack the base. In order t prevent this, a three-point support isused, as shown in Fig. 2, so that the transformer may be suitablymounted without the usual danger of breakage of the insulator body.

Having described my invention in what I now consider to represent apreferred embodiment, I desire to have it understood that the specificstructure described is merely illustrative and that the invention may becarried out by other means.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A transformer comprising a primary winding, a rigid tubularinsulating body around said winding, a magnetic core surrounding a partof said body, said part of said body having an inner surface relativelyclosely fitting said primary winding and the remainder of the innersurface of said body being spaced from said primary winding so as tosubstantially prevent contact between said winding and said remainder ofthe inner surface of said body, means including integral portions ofsaid body supporting said core around said body, means including certainof said integral portions forming a supporting base for saidtransformer, and means including said magnetic core structure relativelyclosely ntting said body part for bracing said part and for absorbingforces due to said primary winding upon short circuit.

2. A transformer comprising a primary winding o! a plurality of turns, abody of solid and relatively brittle insulating material surrounding atleast a portion of said winding, a core structure arranged around saidbody, a terminal for said iwinding, a member surrounding said windingand closely tting the inner walls of said body for supporting saidterminal.

3. A transformer comprising a hollow body of solid and relativelybrittle insulating material, a primary winding of a plurality of turnsarranged in said body, a core structure arranged around said body, anopening in said body, a terminal for said winding, a tubular membersecured to said terminal and extending into said body through saidopening, said member tting closely with the internal walls of said body.

4. A current transformer comprising a substantially U-shaped tubularinsulating body, a primary winding arranged in the passage of saidtubular body, means including terminals for said primary winding mountedon the upper ends of the U legs of said tubular body, said terminalshaving integral portions extending around said winding and downwardlyinto the U legs of said insulating body.

5. A transformer comprising a hollow body of solid and relativelybrittle insulating material, a primary winding of a plurality of turnsarranged in said body, an opening in said body, a terminal for saidwinding, a tubular member secured to said terminal extending into saidbody through said opening, and elastic means closely fitting betweensaid tubular member and the internal walls of said body.

